The preparation of water-absorbent polymers is, for example, summarized in “Modern Superabsorbent Polymer Technology” F. L. Buchholz and A. T. Graham, Wiley-VCH, 1998, or in Ullmann's Encyclopedia Industrial Chemistry, 6th ed. vol. 35 pp 73-103. The preferred preparation process is the solution or gel polymerization. When using this technology usually a monomer mixture is prepared which is discontinuously neutralized and then transferred into the polymerization reactor and is then discontinuously or continuously polymerized to form a polymer gel which is in case of a stirred polymerization comminuted. The polymer gel is subsequently dried, ground, and sieved, and optionally a surface treatment is applied.
Methods of continuous polymerization are, for example, described in WO-A-01/38402, WO-A-03/004237, WO-A-03/022896 and WO-A-01/16197.
WO-A-03/022896 discloses a continuous process for the preparation of superabsorbent polymers in a reactor system comprising at least three zones, wherein the first zone is an initiation zone to which an aqueous monomer mixture comprising an initiation system is fed under conditions to initiate the polymerization, the second zone is a gel-phase zone, and the third zone is a granulation zone. According to a preferred embodiment the reactor is operated under vacuum and any water contained in the vacuum line is condensed and the condensate is recycled to the third zone to enhance the flowability of the polymer gel in the third zone.
EP-A 1 470 905 discloses a method for disintegrating a hydrate polymer by disintegrating a hydrate polymer having a solid content in the range of 50-70% by weight with a screw extruder while supplying 0.1-30 parts by weight of water per 100 parts of the polymer to the extruder. Since the product of disintegration is prevented from inducing mutual adhesion, it does not form masses of conglomeration when it is dried.
Although the measures suggested in the above discussed prior art references lead to some improvements in the handling of the superabsorbent gel polymer in the drying process, there is still a need in the industry to further improve processability of superabsorbent polymer gels in a process for the preparation of superabsorbent polymers. Thus, the object of the present invention is to provide a process for the preparation of superabsorbent polymers, whereby the flowability of the polymer gel in further improved.